(320 ± 0.001 g) to measure weight of the liquid flowing from the
test section outlet over a sufficient time, whereas the flow rates
of gases were measured by three sets of rotameters within the
range of 5–50, 50–500, and 200–2500 cm3
/min, respectively. An
air–water y-shaped mixer served to introduce fluids smoothly
along the test section. Thermocouples and pressure transducers
were installed at various positions to monitor the flow condition
of the working fluids.
Fig. 2 illustrates an exploded view of the test section. The 21
parallel rectangular micro-channels of 0.45 0.41 mm
(width depth) with a 0.54 mm wall thickness between each
channel were fabricated on the copper block in which a cartridge
heater with adjustable input power was installed. The length of
the micro-channels was 40 mm. On the top of the test section,
the cover plate made of polycarbonate was placed to allow optical
access for flow visualization. The copper block was well insulated
by G10 epoxy. Metal plates and the insulators were bolted together
with the copper block and the cover plate to firm up the assembly.
The cover plate, which is transparent, served as a viewing window
for flow visualization. The detailed formation of flow pattern
was registered by precise Stereozoom microscope mounted together
with a camera having shutter speeds of 1/15–1/10,000 s.
An adjustable LED light source was placed perpendicular to the
viewing section.
A set of 14 K-type thermocouples was embedded in the copper
block. For each side of the test section presented in Fig. 2, there
were seven thermocouples inserted at equal distances of 16 mm
along the channel length and 4.3 mm along the direction perpendicular
to the channel length. These temperature measurements
were done to determine surface temperatures at six positions
using linear extrapolation. The arithmetic mean of these surface
temperatures was used to calculate the average heat transfer coef-
ficient. In addition, T-type thermocouples were installed at the inlet
and outlet of the test section to measure the fluid temperature.
All thermocouples and relevant instruments installed in the experimental
apparatus were well calibrated.
To explore different flow phenomena in micro-channels, different
inlet sections were used in the experiments, as shown in Fig. 3.
The inlet section, presented in Fig. 3a, led the gas–liquid mixture
directly to the micro-channels. For the other, as illustrated in
Fig. 3b, the mixture was led to flow through a piece of foamed
plastic polymer, a porous material, and subsequently entered the
micro-channels.
In this work, the experiments for each inlet section were conducted
so the air flow rate increased by small increments while
the water flow rate was constant at the desired value. To keep
the flow from boiling conditions, the heat load of 80 W was provided
by a DC power supply during the experiments. The system
was allowed to approach a steady state before the flow pattern
and relevant data are recorded. The uncertainties associated with
different parameters are given in Table 1.